WASHINGTON--Republican presidential candidate Jeb Bush accused President Barack Obama on Thursday of not ensuring Iraq remained stable after his brother left a "fragile but secure" environment, returning to the topic of Iraq for the second time this week.

At a national security forum in Davenport, Iowa, Bush rejected Democratic criticism that he is trying to rewrite history by saying Obama's policies, not the 2003 U.S.-led invasion of Iraq launched by then-President George W. Bush, contributed to the rise of the Islamic State militant force.

As he did on Tuesday, Bush said Obama and by extension then-Secretary of State Hillary Clinton, the front-running Democratic presidential candidate, bungled a chance to leave behind a contingency force of about 10,000 troops when American forces left Iraq in 2011. Such a force would have helped protect the hard-fought gains made by a U.S. troop surge in 2007, Bush said.

Bush said people have every right to criticize his brother's decision to go to war on faulty claims that Iraq possessed weapons of mass destruction, but in 2009 when Obama took over from George W. Bush, Iraq was "fragile but secure."

"That is a fact and you can't take that away from anybody and you can't rewrite history in that regard," said Bush, a leading contender for the 2016 Republican presidential nomination.

The former Florida governor said it would be impossible to know whether Islamic State would have surfaced without the 2003 invasion. Still he said, "taking out (then-Iraqi President) Saddam Hussein turned out to be a pretty good deal."

Bush has come under sharp criticism this week from Democrats who have accused him of learning nothing from his brother's experience in Iraq. "What was made clear today is that under Jeb, we should expect another four years of the Bush Doctrine," said Democratic National Committee spokeswoman Holly Shulman.

Bush on Tuesday advocated a more muscular approach to Islamic State, saying if elected in November 2016, he would let some U.S. military personnel serve as "spotters" to help identify enemy targets and create a no-fly zone over Syria. Polls show national security is high on the list of priorities for Republican voters as they contemplate a field of 17 Republican candidates.

Bush also said he would keep open the U.S. military prison at Guantanamo Bay, Cuba. Obama is attempting to close it by the time he leaves office.